The present invention pertains to a construction apparatus that is capable of vertical and/or horizontal adjustment, and more particularly to a control for establishing the height and/or lateral displacement of such a tool.
Mechanisms are known which assist an operator of a construction implement in following a reference guide. For example, mechanical elements, which are adapted for contacting a reference string, are mounted on the construction implement tool, such as a road grader blade, in order to provide an indication of the position of the string relative to the blade to control the elevation of the blade. However, these mechanical elements physically displace the string and consequently do not reliably indicate the height of the tool relative to the string. Additionally, the mechanical elements are prone to falling off the string and require manual replacement on the string.
An improvement over the mechanical element is an ultrasonic transducer. In addition to a string, these sensors may be used with a curb, or the road bed, as a reference surface. For example, U.S. Pat. No. 4,924,374 issued to Middleton et al., entitled METHOD FOR AUTOMATIC POSITION CONTROL OF A TOOL, discloses an ultrasonic sensor and control apparatus for a construction implement. The ultrasonic transducer measures the time it takes for an acoustic pulse to travel from the transducer to the reference surface and back to the transducer. Although these sensors provide an improvement over the mechanical sensing element, the ultrasonic sensor only measures the time that it takes for a pulse to travel from the sensor to the reference surface and back, and accordingly only measures the absolute distance to the reference surface and not the true elevation of the blade. If the sensor is laterally shifted from a reference string, there may be a large error introduced in the measurement. The control system used with the ultrasonic system will adjust the height of the blade until the distance between the ultrasonic transducer and the reference guide is at the preset distance. However, because the distance is a function of both lateral and vertical displacement, the control system will adjust the blade to an incorrect elevation.
Construction implements are known which include position control mechanisms. For example, some road graders include control mechanisms for the grader blade which may be used to mechanically adjust the lateral position of the blade relative to the grader. For example, U.S. Pat. No. 4,733,355 entitled NON-CONTACTING RANGE SENSING AND CONTROL DEVICE shows a grader including a sensor which measures the lateral distance from a curb with any resulting error being used to laterally reposition the blade. However, the reference does not provide a practical scheme for controlling the blade height and lateral position with respect to a curb. Further, the reference does not show a practical scheme for following a string.
Ultrasonic sensors require calibration to compensate for changes in the velocity of the acoustic pulse resulting from changes in air conditions such as temperature and humidity. The increases and decreases in velocity cause inaccuracy in the control system because the distance measured is a function of velocity. Various methods have been developed to overcome this problem. For example, the Middleton et al. patent, mentioned above, includes a thermister, which senses variations in temperature and compensates for variations in temperature automatically. However, the thermister does not provide an indication of all the air parameters which affect the velocity of the acoustic pulse such as humidity. Another known system for compensating for changes in pulse velocity includes a reference surface which is spaced a predetermined distance directly in front of an ultrasonic transducer. The time that it takes a pulse from the transducer to travel to the fixed reference surface and back is continuously measured, and the measured time is used to compensate for variations in the air conditions. However, if such a calibration reference surface is used with a construction implement it interferes with the monitoring of the actual reference surface, which must be accurately monitored to control the height of the blade, and is vulnerable to damage.